The present invention relates to an arrangement for checking the resistance of a load, preferably the squib of an air bag, connected to the secondary winding of a transformer.
An arrangement for checking the squib resistance on the secondary side of a rotary transformer is described in German Patent No. 38 12 633 A1. According to this related art, a rotary transformer is used, for example, for signal transmission between a controller and an air bag built into the steering wheel of a vehicle. The functionality of an air bag must be tested on an ongoing basis to ensure that it is ready to operate in an emergency situation. For this purpose, the resistance of the squib, which should typically be between 1.8 xcexa9and 2.5 xcexa9, is periodically measured. The transmission of signals using inductively coupled windings of the rotary transformer depends on the distance between the two windings, i.e., the size of the gap between the pot-type cores of the windings. Assembly and manufacturing tolerances of the steering wheel, on which the secondary-side pot-type core of the rotary transformer is installed, can distort the measured resistance value of the squib. In order to avoid such measuring signal distortions, the aforementioned related art provides an oscillating circuit on the secondary side of the rotary transformer, which is excited by a control signal from the primary side. After the control signal is switched off, the decaying response signal of the oscillating circuit is retransmitted back to the primary side and the secondary side resistance is obtained from the time constant of the decaying response signal.
In order to check the condition of a squib for a safety system (e.g. air bag) connected to the secondary side of a transformer, according to German Patent No. 38 12 631, a measuring resistance equal to the resistance of the squib can be connected to the secondary side. Both the squib and the measuring resistance can be alternately connected and disconnected, independently of each other, by switches on the secondary side of the transformer. Squib and measuring resistance are connected and disconnected by the switches. The voltage with the squib connected and the voltage with the measuring resistance connected are measured on the primary side of the transformer and the two voltages are compared. When no error is present, the two voltages should be equal because of the same resistance values of the squib and the measuring resistance. An error is signaled in the case of a deviation.
In this known measuring process, the squib is periodically disconnected from the secondary side of the transformer. Thus there are phases where in the case of an accident no pulse can reach the squib, and triggering is thus prevented.
An object of the present invention is therefore to provide an arrangement for checking the resistance of a load, preferably the squib of an air bag, connected to the secondary side of a transformer, where the function of the load is not interrupted during the checking procedure. SUMMARY OF THE INVENTION
According to the presume invention, the resistance of a load, preferably the squib of an air bag, connected to the secondary side of a rotary transformer, is checked using a well-defined measuring resistance connected to the load. The change in the secondaryside total resistance due to the measuring resistance connected to it is recorded and, if the change in resistance differs from a predefined value, a signal is generated.
The measuring principle used for checking a load connected to the secondary side of a transformer requires only one measuring resistance additionally provided on the secondary side of the transformer, which may be connected to the load. Thus, the arrangement can be implemented with a very simple circuit.